SDDM hangs at boot on my computer so I have to set it to auto login to get to my desktop. That's not particularly secure, so I would like as a workaround the screen locker to autostart. How might I achieve this in Plasma 5?

Cheers!

Try this:
System Settings -> Startup and Shutdown.
Then "Add Program" and in the text box, type:
/usr/bin/loginctl lock-session
and press Ok.
(this may be /usr/sbin/ on your installation)

Thanks! The file is located somewhere else on my distro (Kubuntu 15.10), but still somewhere in the path variable. (It's in neither /usr/bin or /usr/sbin.)

'which loginctl' should show you location. it is better for security to put full path to system commands when running them automatically in the background. this eliminates anything injecting variables into the PATH variable.

Thanks again.

I almost prefer this method of security over an sddm password/user prompt since I'm the only user on this machine, running only a single DE. Once you enter the password, your desktop is fully running, and because my machine is snappy enough, it loads the lockscreen so fast you don't get a chance to see the desktop.